The present invention generally relates to apparatus and methods for temperature controlling a conditioned space and more particularly relates to temperature controlling systems which utilize a cryogen.
It has been known for some time to temperature condition an enclosed space for the purpose of transporting temperature sensitive materials, such as food stuffs. The most prevalent current approach is to cool and/or heat a transportable conditioned space (e.g. a refrigerated truck, trailer, or rail car) with a mechanical, condensation/evaporation system utilizing a fossil fuel powered compressor.
Unfortunately, many such mechanical systems employ refrigerants of the chlorofluorocarbon (CFC) family, because of the desirable heat of vaporization and temperature/pressure vaporization points. Certain studies have indicated that such refrigerants may produce undue deterioration of the earth's ozone layer. In response thereto, various laws and regulations have been enacted to control the release of such refrigerants to the atmosphere.
A relatively new and exciting alternative to mechanical systems utilizing CFC refrigerants is a temperature conditioning system based upon the controlled energy release from a transportable store of cryogenic liquid. In the most environmentally acceptable approaches, this involves the use of a liquified inert gas, such as nitrogen or carbon dioxide, which may be simply and harmlessly exhausted into the atmosphere at ambient temperature and pressure, after the cooling potential in its cryogenic state has been utilized to provide temperature conditioning of the controlled space.
Ideally, the entire cryogenic temperature control system is powered to the greatest extent possible by the release of the pressure stored by the cryogenic liquid with minimal or no additional energy sources. This highly integrated design promotes reliability, low cost of manufacture, and freedom from acoustic and chemical pollution.
Control valves, for example, are preferably powered by cryogenic energy rather than outside electrical or other energy sources. Similarly, attempts to provide mechanical power from the cryogenic fluid have been greatly enhanced through the use of vapor powered motors. However, such conversions of cryogenic energy to mechanical energy must be accomplished in the most efficient manner possible to prevent premature depletion of the cryogenic liquid energy source. Whereas great strides have been made concerning the design of the individual components, efficiency of cryogenic liquid energy usage is also a matter of system level design.
For example in prior art approaches, the vapor motor is powered by the vapor retrieved from the low pressure end of the evaporation coils. Whereas this is a particularly efficient method for providing ventilation to the evaporation coils during continuous operation, at system start-up there may be substantial delay in the arrival of vapor to the vapor motor thus promising clogging of the evaporation coils with dry ice and uneven evaporation.